Indoor radon and thoron in dwellings and workplaces across three contiguous geological formations in Southwest Nigeria
摘要
The first regional survey of concurrent indoor radon and thoron measurements in Southwestern Nigeria was organized using the passive radon-thoron discriminative detectors RADUET. The measurements were carried out in various dwellings and workplaces across three geological formations within the Southwestern Nigeria basin, which consists of recent sediments in Akoka (Lagos State), cretaceous sediments known as Abeokuta formation in Ilishan (Ogun State), and basements in Alabata (Abeokuta, Ogun State). Both radon and thoron concentrations at 193 sites showed log-normal distributions, with significantly (p < 0.05) higher values of thoron than radon concentrations and a weak correlation (R = 0.224) between the two. The ranges and arithmetic means of the concentrations were 6–132 Bq m−3and 24 ± 21 Bq m−3 for radon and 2–709 Bq m−3 and 94 ± 124 Bq m−3 for thoron, respectively. ANOVA results showed significant variations in radon and thoron concentrations according to the underlying geology, with radon concentrations in Ilishan (cretaceous sedimentary) significantly (p < 0.05) higher than those of Akoka (recent sedimentary) and Alabata (basement complex). No significant differences (p = 0.09) were found between thoron concentrations in Alabata and Ilishan, and both locations had significantly higher (p < 0.05) concentrations of thoron than those recorded in Akoka. A test of the influence of building types showed that radon and thoron concentrations in offices were significantly (p < 0.05) lower than those in homes. None of the thoron concentrations were zero, and almost all were higher than the corresponding radon concentrations. Some of the radon concentrations exceeded the recommended reference level of 100 Bq m−3, but all were below the action level of 300 Bq m−3. This study has shown that with high concentrations of thoron, its contributions should not be neglected in indoor radon measurements, particularly in the areas with old sedimentary and basement complex geology. It is noted that dose evaluation is required to quantify the relative contributions of radon and thoron to human exposures in the three locations. Meanwhile, occupants of buildings in the study areas should be encouraged to optimize indoor ventilation.